Appendix B Methodology for Year 9 of the NDNS RP

B.1 Overview of methodology

This chapter provides an overview of methodology for Year 9 of the NDNS RP. An overview of the methodology, sample design and response rates for Years 1 to 8 is provided in previous reports. Most aspects of the survey methodology remained consistent across fieldwork years; key changes are discussed in the respective reports.¹,²,³

The NDNS RP has run continuously since 2008 and is a cross-sectional survey, designed to be representative of the UK population. In order to meet the aims of the survey (see Introduction to main report) a sample of people representative of the UK population aged 1.5 years and over was required. This sample was drawn from the Postcode Address File (PAF)⁴ a list of all the addresses in the UK. In order to improve cost effectiveness, the addresses were clustered into Primary Sampling Units (PSUs), small geographical areas, based on postcode sectors, randomly selected from across the UK. A list of addresses was randomly selected from each PSU.

Information describing the purpose of the survey was posted to all selected addresses. This was followed by a face-to-face visit by an interviewer to each address to recruit participants in the eligible age range(s). The survey aims to collect data from a UK representative core sample of 1,000 people per year, 500 adults (aged 19 years and over) and 500 children (aged 1.5 to 18 years). In order to achieve (as far as possible) equal numbers of adults and children in the sample, at some addresses only children were selected to take part. Extra addresses were selected in Wales and Northern Ireland to boost the sample size in these countries and enable comparisons to be made between the UK countries.

At each address, the interviewer enumerated the number of households and, in cases where there were 2 or more, randomly selected one for the NDNS

¹ The sample was drawn from the ‘small users’ sub-file of the Postcode Address File (PAF), a computer list, prepared by the Post Office, of all the addresses (delivery points) which receive fewer than 25 articles of mail a day.

National Diet and Nutrition Survey Years 1 to 9 of the Rolling Programme (2008/2009 – 2016/2017): Time trend and income analysis RP. From each selected household the interviewer randomly selected up to 1 adult and 1 child to take part in the survey. These are known as participants.

The first stage of the survey comprised a face-to-face Computer Assisted Personal Interview (CAPI) with each participant (or in the case of a young child, their parent or guardian(^b)), completion of an estimated 4-day food diary by the participant, measurements of height and weight and collection of a spot urine sample (aged 4 years and over).

The interviewer also collected information on shopping and food preparation practices and facilities in the household by additionally interviewing the Main Food Provider (MFP)(^c) of the household where this was not a selected participant. The MFP was the person who was best placed to answer questions about food purchased and prepared for the participant(s). The interview also identified the Household Reference Person (HRP)(^d) in each household and asked questions about housing tenure, as well as his or her employment, to determine the socioeconomic classification of the household.(^e)

Participants who took part in the CAPI interview and completed a food diary for at least 3 days were classified as ‘fully productive’ and were invited to take part in the second stage of the survey. This involved a visit from a nurse to take further physical measurements and a blood sample.

(^b) A guardian is defined as a person with legal responsibility for the child. (^c) The Main Food Provider (MFP) is the person in the household with the main responsibility for shopping and preparing food. If these tasks were shared equally between 2 people, for example if 1 person did all the shopping and another person did all the cooking, then either resident could be classified as the MFP. (^d) The ‘Household Reference Person’ (HRP) was defined as the householder (a person in whose name the property is owned or rented) with the highest income. If there was more than 1 householder and they had equal income, then the eldest was selected as the HRP. (^e) Questions were asked to ascertain whether the HRP was in paid work at the time of the interview and, if not, whether they had ever had a paid job. If the HRP had ever worked, there were further questions about their current or most recent job in order to classify HRPs into the National Statistics Socio-economic Classification (NS-SEC) groupings.

**National Diet and Nutrition Survey**

Years 1 to 9 of the Rolling Programme (2008/2009 – 2016/2017): Time trend and income analysis B.2 Sample design

B.2.1 Selecting addresses

The sample was drawn from the PAF. In Year 9, 4,424 addresses were selected from 158 PSUs. This included “country boosts” in Wales and Northern Ireland. The address sample in Year 9 was drawn as follows:

| Sample Type | Number of Addresses | |------------------------------|---------------------| | Core UK sample | 3,360 addresses in 120 PSUs | | Wales “country boost” | 504 addresses in 18 PSUs | | Northern Ireland “country boost” | 560 addresses in 20 PSUs |

In each PSU, 28 addresses were randomly selected. At each address, the interviewer established the number of households and, in cases where there were 2 or more, selected 1 household at random.

B.2.1 Selecting participants

As only around one-third of households contain children, over-sampling of children was required in order to achieve equal numbers of adult (19 years and over) and child (1.5 to 18 years) participants each year. The 28 selected addresses were randomly allocated to 1 of 2 groups to determine whether an adult (aged 19 years or over) and a child (aged 1.5 to 18 years), or a child only, were selected for interview. At 10 of the selected addresses the interviewer selected 1 adult and, where present, 1 child for inclusion in the survey (“basic” addresses). The remaining 18 addresses were for a “child boost” where only a child was recruited, and therefore the interviewer only carried out interviews in households with children. In households containing more than 1 eligible person (adult and/or child), interviewers selected the participant(s) using a random selection procedure.

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1 The sample was boosted in Wales in Years 2 to 9 and in Northern Ireland in Years 1 to 4 and 6 to 9 (i.e. not in Year 5).

National Diet and Nutrition Survey Years 1 to 9 of the Rolling Programme (2008/2009 – 2016/2017): Time trend and income analysis B.3 Ethics approval

Ethics approval for the study was obtained from the Cambridge South NRES Committee (Ref. No. 13/EE/0016). Research governance approval was sought for all participating NHS laboratories and obtained where required by the Research and Development (R&D) Committee for each laboratory.

B.4 Fieldwork

Year 9 fieldwork was issued monthly to interviewers and nurses in the following quarters:

| Interviewers (Stage 1) | Nurses (Stage 2) | |------------------------|------------------| | Quarter 1 | April-June 2016 | | | July-September 2016 | | Quarter 2 | July-September 2016 | | | October-December 2016 | | Quarter 3 | October-December 2016 | | | January-March 2017 | | Quarter 4 | January-March 2017 | | | April-June 2017 |

Stage 1 fieldwork commenced on the first weekday of the month, and interviewers were given 6 weeks in which to complete their assignment. Stage 2 fieldwork for a particular month started 8 weeks after the interviewer deadline. Nurses had up to 7 weeks to complete their work.

B.5 Overview of survey components and fieldwork procedures

The survey is delivered in 2 stages:

Stage 1: Interviewer visit:

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8 This covered Years 6 to 9 of the RP. Ethics approval for Years 1 to 5 was obtained from the Oxfordshire A Research Ethics Committee (Ref. No. 07/H0604/113).

9 The Research Governance Framework is intended to define the broad principles of good research practice, and to ensure that health and social care research is conducted to high scientific and ethical standards.

National Diet and Nutrition Survey Years 1 to 9 of the Rolling Programme (2008/2009 – 2016/2017): Time trend and income analysis • Estimated 4-day food diary • Detailed background interview • Interview with MFP • Height and weight measurements • Smoking and drinking self-completion questionnaires • Physical activity self-completion questionnaire • Collection of spot urine sample

Stage 2: Nurse visit:

• Blood sample • Physical measurements,\\textsuperscript{i} waist and hip, demispan and infant length • Blood pressure • Collection of information about prescribed medicines

B.5.1 Stage 1: the interviewer visits

A letter and leaflet describing the purpose of the survey was sent to all sampled addresses before the fieldwork start date. A few days later, interviewers visited the addresses to determine whether the address was private, residential and occupied. They then carried out the selection process and, for children aged under 16 years, sought both the child’s and their parent’s (or guardian’s) consent to interview.

Interviewers carried out 3 main visits to households who agreed to participate:

• Visit 1: Estimated 4-day food diary explained to the participant and left with them to complete; interviewer-administered CAPI; height and weight measurements; collection of spot urine sample; self-completion booklets in which children and young people were asked to record their smoking and drinking habits. Participants aged 16 years and above were asked to fill in a self-completion questionnaire designed to collect information about physical activity (the Recent Physical Activity Questionnaire (RPAQ)).\\textsuperscript{j,k}

\\textsuperscript{i} See section B.5.2 for age ranges for each physical measurement. \\textsuperscript{j} Based on the Recent Physical Activity Questionnaire developed by the MRC Epidemiology Unit, Cambridge. \\textsuperscript{k} Children aged under 16 years were asked about physical activity during the CAPI interview.

National Diet and Nutrition Survey Years 1 to 9 of the Rolling Programme (2008/2009 – 2016/2017): Time trend and income analysis • **Visit 2:** The diary check up visit, where the interviewer reviewed the completion of the 4-day food diary so far and filled in any missing information with the participant.

• **Visit 3:** Review and collection of 4-day food diary, RPAQ self-completion and further CAPI questionnaire administration.¹

At the end of the third interviewer visit, interviewers gave each participant completing at least three food diary recording days a token of appreciation (£30 high street gift card). Interviewers then introduced the second stage of the survey, asking for permission for the nurse to visit.

Further details about information collected during the interviewer stage (and the fieldwork documents used) can be found in appendices C to F.

**Computer Assisted Personal Interview (CAPI) programme**

CAPI interviewing involves the interviewer reading questions from a laptop screen and entering the participants’ responses into designated fields. The CAPI questionnaire had 3 main elements: household composition/structure interview, MFP interview and individual interview. The individual questionnaire, asked of each selected participant, had 2 parts: Part 1, which was asked at the first main interviewer visit; and Part 2, which was asked at the third main visit after the interviewer collected the food diary.

The content of the CAPI questionnaires is shown in appendix D.

**Collection of dietary data: the 4-day food diary**

Based on the day of the first individual CAPI interview, the interviewer’s laptop program selected 4 consecutive days as the food diary recording period. Participants were provided with a paper diary and asked to keep a record of everything they ate and drank over these 4 days, both in and outside the home. Interviewers carried out a food diary check visit with participants on the second or third day of recording either in person or, where a visit was not possible, over the telephone, with the aim of collecting missing detail for foods recorded, improving recording for the remaining days and also providing encouragement to participants to continue recording. Interviewers then

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¹ Height and weight measurements and spot urine collection could also be carried out at this visit if not possible at visit 1.

**National Diet and Nutrition Survey**

Years 1 to 9 of the Rolling Programme (2008/2009 – 2016/2017): Time trend and income analysis returned to collect the diary and check the remaining days no later than 3 days after the final day of recording.

As participants were not expected to weigh their food and drink, portion sizes were estimated using household measures (e.g. 2 thick slices of bread, 4 tablespoons of peas) or using weights from labels (e.g. 420g tin of baked beans, 330ml can of lemonade). Those aged 16 years and over were also able to describe their portion size using photographs of 10 frequently consumed foods reproduced in the diary. For those aged under 16 years, age-appropriate food photograph atlases were used. The atlases presented a range of served and leftover portion sizes for 10 commonly consumed foods for which portion size estimation is difficult. Most photographs could be used to estimate amounts for other foods in addition to the actual foods shown e.g. photos of rice could also be used for couscous. These alternatives were listed on a separate card - the Equivalent Foods List (EFL). When reviewing the diaries, interviewers asked participants to select the appropriate portion sizes for all diary entries represented in the atlas.

A parent was asked to keep the food diary on behalf of participants aged 11 years and younger, with the child contributing information where possible and with help from other carers.

Appendix A provides full details of the dietary data collection and processing protocols.

Selection of food diary start day

The study design aimed to give an even representation of diary days on all days of the week so the food diary could start on any day of the week and run for 4 consecutive days. The diary start day for each participant was assigned by the CAPI program but could be changed by the interviewer if the participant preferred a different day.

Results in this report are based on dietary assessment using a 4-day estimated food diary and represent a daily average of the days assessed. The survey is designed so that all days of the week are evenly represented.

Participants with dietary data for at least three days were included in the analyses.

National Diet and Nutrition Survey Years 1 to 9 of the Rolling Programme (2008/2009 – 2016/2017): Time trend and income analysis Spot urine sample

Spot urine samples were collected in order to provide an assessment of iodine concentrations in the UK population. Participants aged 4 years and over were asked to provide a small sample of urine in a sterile container (a ‘spot urine sample’) after reading the relevant information leaflets and providing written consent (see appendices C and H).n

B.5.2 Stage 2: the nurse visit

Stage 2 of the survey was carried out by a qualified nurse and took place within 2 to 4 months of the interviewer stage. All individuals completing 3 or 4 food diary days were eligible for a nurse visit.

At the end of Stage 1, interviewers provided participants with information leaflets giving details of the nurse visit and sought agreement for the nurse to make contact. Nurses could provide these again if necessary. The nurse asked questions about prescribed medications before taking, with agreement, a number of physical measurements.

Measurements taken by the nurse

A summary of the information collected during the nurse stage is provided below. Some of the information collected by nurses was limited to particular age groups.

| Measurement or procedure | Participant | |-------------------------------------------|----------------------------------| | Details of prescribed medications | All ages | | Blood pressure | Aged 4 years and over | | Infant length measurement | Aged 18 to 23 months |

n Individuals using a urinary catheter, menstruating females and children aged 4 and over who are not dry during the day were excluded from providing a sample.

National Diet and Nutrition Survey Years 1 to 9 of the Rolling Programme (2008/2009 – 2016/2017): Time trend and income analysis Waist and hip circumferences Aged 11 years and over

Demispan Aged 65 years and over and those aged 16 to 64 years where height could not be measured

Non-fasting blood sample Aged 1.5 to 3 years and people with diabetes not willing to fast

Fasting blood sample Aged 4 years and over

The nurse fieldwork documents are provided in appendices G and H. Measurement protocols are provided in appendix L.

**Blood sample**

After providing the physical measurements, participants were asked whether they were willing to give a small blood sample by venepuncture after an overnight fast (those aged 1.5 to 3 years and people with diabetes not willing to fast were asked whether they were willing to provide a non-fasting blood sample). The nurse obtained written consent from the participants aged 16 years and over before the sample was taken. For children aged 1.5 to 15 years, written consent of a parent or guardian was required and nurses additionally obtained written assent from the child where possible. For those aged 10 years or younger, blood was taken by a paediatric phlebotomist who accompanied the nurse on the visit. Nurses also sought written agreement to store part of the blood sample for additional analyses at a future date. Participants who provided a blood sample were given a £20 high street gift card as a token of appreciation for agreeing to this part of the study.

**B.6 Feedback to participants and GPs**

Participants who completed 3 or 4 food diary recording days were asked whether they would like to be sent feedback on the analysis of their diary and how this compared to nutrient intake recommendations. The feedback also included general information on sources of healthy eating advice. Further

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- Nurses qualified and experienced in paediatric phlebotomy took blood samples from children. Protocols and procedures are in line with official RCN guidelines.

**National Diet and Nutrition Survey**

Years 1 to 9 of the Rolling Programme (2008/2009 – 2016/2017): Time trend and income analysis information about the dietary feedback can be found in appendix A and an example of the dietary feedback is provided in appendix J.

Each participant was also given a ‘Measurement Record Card’ on which the interviewer and nurse recorded the person’s height, weight, body mass index (BMI) (if aged 16 years and over), blood pressure (if aged 4 years and over) and other age-dependent anthropometric measurements: waist and hip circumferences (if aged 11 years and over); demispan measurement (if aged 65 years and over) and infant length (if aged 18 to 23 months). Participants who provided a blood sample were asked whether they wished to be sent results of the blood sample analyses most related to their health. Participants were also asked if they wanted details of these analyses, their BMI and their blood pressure readings to be sent to their GP. If an individual wished to receive results (or have them sent to their GP), written consent was obtained from them (or from the parent/guardian in the case of a child). See appendix J for an example of feedback.

B.7 Fieldwork quality control

B.7.1 Project specific training for interviewers and nurses

Fieldwork in England, Scotland and Wales was carried out by NatCen’s panel of interviewers and nurses. In Northern Ireland, fieldwork was carried out by interviewers and nurses working for the Northern Ireland Statistics and Research Agency (NISRA).

All interviewers and nurses working on the NDNS RP were briefed and trained before undertaking an assignment and were monitored during their assignment. Fieldworkers were also issued with comprehensive written instructions covering survey procedures and measurement protocols.

B.7.2 Training for interviewers

All new-to-NDNS RP interviewers (and those who had not worked on the study in the previous fieldwork year) attended a 1.5-day training course where they were fully briefed on the protocols and administration of the survey. Interviewers who had worked on the previous survey year attended a 1-day refresher briefing. The full and refresher briefing sessions covered background and content, doorstep approach, questionnaire administration (including practice sessions), placement and collection of self-completions and the placement, checking and collection of the 4-day food diaries. All new interviewers and those who had not worked on the survey in the preceding 12 months were accredited on taking consistent, accurate height and weight measurements.

After the briefing, “early work” checks were carried out on the first 2 or 3 food diaries returned by each interviewer with timely feedback provided on any areas of concern. Before working on a second or subsequent assignment, all interviewers received feedback on the diaries from their previous assignment.

B.7.3 Training for nurses

Nurse briefings lasted 1 day and covered equipment training, blood sampling and questionnaire administration (including practice sessions). Nurses were also briefed on the demispan, waist and hip and infant length measurement protocols. All other physical measurements were either regularly taken by nurses on the NDNS RP and other NatCen surveys or the newer nurses attended a general training session which covered these protocols. New nurses were accredited during the briefing. This provided training and assessment on taking consistent and accurate waist and hip and demispan measurements. Existing nurses who did not hold a valid demispan or waist and hip measurement accreditation attended a separate ‘accreditation’ session.

Nurses who had a gap of 3 months or more between assignments and new-to-NDNS RP nurses completed three homework exercises (covering blood sample tube labelling, and consents/blood despatch) which were marked and individual feedback given to each nurse.

B.8 Weighting the survey data

It is necessary to apply weighting factors to the data collected in the NDNS RP for 2 reasons: to remove any bias in the observed results which may be due to differences in the probability of households and individuals being selected to take part; and to attempt to reduce non-response bias. The survey was designed so that no more than 1 adult and 1 child were selected from any 1 household to take part. This meant that adults living in households with 1 or more other adults, and children in households with 1 or more other child were less likely to be selected than were adults or children in single adult/child households.

In addition, the multi-stage design means there were a number of stages in the survey where it was possible for participants to drop out. If the people who refused to participate at a particular stage were systematically different from those who took part then the sample would be biased.

Weighting factors were used to correct for both these cases. There were 2 stages to the weighting scheme: the first was to generate a set of design weights to correct for the unequal selection probabilities; and the second was to create a set of weights to adjust for non-response. The final weights were a product of the selection weights and the non-response weights.

The sample design includes an adjustment for selecting more addresses in Wales and Northern Ireland. All of the addresses in these countries, and therefore participants, are weighted down as a result. The applied weights puts the 4 countries into their correct population proportions in the UK so that, for example, the percentage of the NDNS RP sample in Wales is the same as the percentage of the UK population that is in Wales.

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1 National Diet and Nutrition Survey: Results from Years 1,2,3 and 4 (combined) of the Rolling Programme (2008/2009 – 2011/2012). [Internet]. Available from: www.gov.uk/government/statistics/national-diet-and-nutrition-survey-results-from-years-1-to-4-combined-of-the-rolling-programme-for-2008-and-2009-to-2011-and-2012 2 National Diet and Nutrition Survey: Results from Years 5 and 6 (combined) of the Rolling Programme (2012/2013 – 2013/2014). [Internet]. Available from: www.gov.uk/government/statistics/ndns-results-from-years-5-and-6-combined 3 National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015 – 2015/2016). [Internet]. Available from: www.gov.uk/government/statistics/ndns-results-from-years-7-and-8-combined 4 Ref: RCN (2013). An education and training competence framework for capillary blood sampling and venepuncture in children and young people, RCN London. ISBN 978-1-908782-56